Pneumatic tire

ABSTRACT

A pneumatic tire reducing road noise without deteriorating durability and rolling resistance. In this pneumatic tire, the bead filler includes a triangle portion abutting a bead core and having a substantially triangular cross-sectional shape, and a uniform-thickness portion extending from a top of the triangle portion in a radially outward direction of the tire and having a substantially uniform thickness. An upper-end position of the uniform-thickness portion is outward, in a radial direction of the tire, from a position 1.6 times as high as the height of the triangle portion, and is at least 5 mm apart from the belt layers. A thickness of the uniform-thickness portion is thicker than 0.1 mm and thinner than 0.5 times a lower-end width of the triangle portion. The bead filler includes a plurality of rubber layers formed of mutually different rubber compositions. The rubber compositions forming these rubber layers have loss tangents (tan δ) gradually decreasing from the outermost-periphery rubber layer toward the innermost-periphery rubber layer, are harder than other rubber compositions neighboring the bead filler, and have breaking elongations whose mutual differences are 50% points or less.

TECHNICAL FIELD

The present invention relates to a pneumatic tire provided with a beadfiller extending from a bead portion to a sidewall portion. Morespecifically, the present invention relates to a pneumatic tire allowedto reduce road noise without deteriorating durability and rollingresistance.

BACKGROUND ART

In a pneumatic tire, in order to ensure driving stability, what isnormally performed is to bury, from a bead portion to a sidewallportion, a bead filler formed of a hard rubber composition. Furthermore,there has been proposed an approach to improve a variety of tireperformances by devising cross-sectional shapes of the bead filler.

For example, there has been proposed an approach where: an increase incircumferential stiffness and a reduction in longitudinal stiffness arecompatibly achieved by elongating a cross-sectional shape of the beadfiller, and as a result, road noise having frequencies around 40 Hz andaround 100 Hz is reduced (refer to Patent Document 1, for example). Inthis case, in compliance with noise regulations having been stiffened inrecent years, it has been required to make the bead filler still thinnerand still higher.

However, in a case where a bead filler formed of a hard rubbercomposition is buried from a bead portion to a sidewall portion and ismade high enough to be extended to a vicinity of a shoulder portion,there arises a problem that rolling resistance and durability aredeteriorated.

[Patent Document 1] Japanese patent application Kokai publication No.Hei8-276713

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a pneumatic tireallowed to reduce road noise without deteriorating durability androlling resistance.

A pneumatic tire according to the present invention for achieving theabove object is a pneumatic tire including: a carcass layer mountedbetween a pair of a left and a right bead portions; belt layers arrangedon an outer periphery of the carcass layer located in a tread portion;and a bead filler arranged along the carcass layer on an outer peripheryof a bead core in each of the bead portions. The pneumatic tire ischaracterized in that: the bead filler includes a triangle portionabutting the bead core and having a substantially triangularcross-sectional shape, and a uniform-thickness portion extending from atop of the triangle portion in a radially outward direction of the tireand having a substantially uniform thickness; a position of an upper endof the uniform-thickness portion is outward, in a radial direction ofthe tire, from a position 1.6 times as high as a height of the triangleportion, and is at least 5 mm apart from the belt layers; a thickness ofthe uniform-thickness portion is thicker than 0.1 mm and thinner than0.5 times a width of a lower end of the triangle portion; the beadfiller includes a plurality of rubber layers respectively formed ofmutually different rubber compositions; plural kinds of the rubbercompositions forming these rubber layers have loss tangents (tan δ)gradually decreasing from the rubber layer at the innermost periphery ofthe rubber layers toward the rubber layer at the outermost peripherythereof, are harder than other rubber compositions neighboring the beadfiller and have breaking elongations whose mutual differences are 50%points or less.

By thus defining the cross-sectional shapes of the bead filler and by,while providing a plurality of rubber layers respectively formed ofmutually different rubber compositions, defining physical properties ofplural kinds of the rubber compositions forming the rubber layers, itbecomes possible to reduce road noise without deteriorating durabilityand rolling resistance.

In the present invention, in order to obtain preferable improvements inthe abovementioned performances, it is preferable that the rubbercomposition of at least one rubber layer (in particular, the rubberlayer at the outermost periphery) among the plurality of rubber layersincluded in the bead filler, except for the rubber layer at theinnermost periphery, have a loss tangent (tan δ) of 0.01 to 0.25 whenmeasured at a temperature of 60° C., a JIS-A hardness of 70 to 95 whenmeasured at a temperature of 23° C., and a breaking elongation not lessthan 200% when measured at a temperature of 23° C. in a tensile test.

Additionally, in order for the above physical properties to beexhibited, it is preferable that the rubber composition of at least onerubber layer (in particular, the rubber layer at the outermostperiphery) among the plurality of rubber layers included in the beadfiller, except for the rubber layer at the innermost periphery, be arubber composition obtained by blending 20 to 120 weight parts of silicaand 0 to 60 weight parts of carbon black with 100 weight parts ofrubber. In particular, it is preferable that the bead filler beconstituted of two rubber layers respectively formed of mutuallydifferent rubber compositions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a half cross-sectional view showing a pneumatic tire accordingto an embodiment of the present invention.

FIG. 2 is a half cross-sectional view showing a pneumatic tire accordingto another embodiment of the invention.

FIG. 3 is a half cross-sectional view showing a pneumatic tire accordingto still another embodiment of the invention.

FIG. 4 is a half cross-sectional view showing a pneumatic tire accordingto still another embodiment of the invention.

FIG. 5 is a half cross-sectional view showing a pneumatic tire accordingto still another embodiment of the invention.

FIG. 6 is a half cross-sectional view showing a pneumatic tire accordingto still another embodiment of the invention.

FIG. 7 is a half cross-sectional view showing a pneumatic tire accordingto still another embodiment of the invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter, a detailed description will be given of a configuration ofthe present invention with reference to the attached drawings.

FIG. 1 shows a pneumatic tire according to an embodiment of the presentinvention. In FIG. 1, reference numerals 1, 2, and 3 respectively denotea tread portion, a sidewall portion, and a bead portion. A carcass layer4 is mounted between a pair of the left and right bead portions 3, 3,and has an end part folded back from the inside to the outside of thetire around a bead core 5. A plurality of belt layers 6, 6 are buried onthe outer periphery of the carcass layer 4 in the tread portion. Thesebelt layers 6, 6 are arranged in order that cords can tilt to thecircumferential direction of the tire and that the cords can cross oneanother between the belt layers.

On an outer periphery of each of the bead cores 5, a bead filler 7constituted of hard rubber compositions is buried along the carcasslayer 4. This bead filler 7 extends from the bead portion 3 along thesidewall portion 2, and beyond the widest-width position of the tire,reaches the shoulder portion of the tire.

In a cross-sectional view taken along a meridian of the tire, this beadfiller 7 includes a triangle portion abutting the bead core 5 and havinga substantially triangular cross-sectional shape, and auniform-thickness portion extending from a top of the triangle portionin a radially outward direction of the tire and having a substantiallyuniform thickness. Here, the bead filler 7 has a height L measured in aradial direction of the tire by taking an upper end of the bead core 5as a base, and the triangle portion of the bead filler 7 has a height Hmeasured in a radial direction of the tire by taking the upper end ofthe bead core 5 as a base.

An upper-end position A of the uniform-thickness portion of the beatfiller 7 is outward, in a radial direction of the tire, from a position1.6 times as high as the height H of the triangle portion, and has adistance X being at least 5 mm, the distance X being measured along theinner surface of the tire from an edge B of the belt layers 6. If theupper-end position A of the uniform-thickness portion of the bead filler7 is inward, in a radial direction of the tire, from the position 1.6times as high as the height H of the triangle portion, road noise havinga frequency around 40 Hz is deteriorated. To the contrary, if thedistance X becomes less than 5 mm by having the upper-end position Aexcessively close to the edge B, rolling resistance is deterioratedwhile road noise having a frequency around 100 Hz is deteriorated.

A thickness T of the uniform-thickness portion of the bead filler 7 isthicker than 0.1 mm and thinner than 0.5 times a lower-end width G ofthe triangle portion. It is difficult to make the thickness T of theuniform-thickness portion to be 0.1 mm or less in manufacturing, and tothe contrary, if the thickness T of the uniform-thickness portion isthicker than 0.5 times the lower-end width G of the triangle portion,road noise having a frequency around 100 Hz is deteriorated.

The bead filler 7 includes a plurality of rubber layers formed ofmutually different rubber compositions. Plural kinds of the rubbercompositions forming these rubber layers have loss tangents (tan δ)gradually decreasing from the rubber layer at the innermost periphery ofthe rubber layers toward the rubber layer at the outermost peripherythereof, are harder than other rubber compositions neighboring the beadfiller 7, and have breaking elongations whose mutual differences are 50%points or less. That is, a loss tangent of the rubber compositionforming a rubber layer 7 a at the outermost periphery is smaller thanthat of a rubber layer 7 b at the innermost periphery. If this relationis turned around, rolling resistance is deteriorated. In addition, adifference between a breaking elongation (in %) of the rubber layer 7 aat the outermost periphery and a breaking elongation (in %) of therubber layer 7 b at the innermost periphery is 50% points or less. Ifthe mutual difference exceeds 50%, durability is deteriorated.

It is preferable that a rubber composition of at least one (inparticular, the rubber layer 7 a at the outermost periphery) among theplurality of rubber layers included in the bead filler 7, except for therubber layer at the innermost periphery, have a loss tangent (tan δ) of0.01 to 0.25 when measured at a temperature of 60° C., a JIS-A hardnessof 70 to 95 when measured at a temperature of 23° C., and a breakingelongation not less than 200% when measured at a temperature of 23° C.in a tensile test.

With regard to the rubber composition forming the rubber layer 7 a atthe outermost periphery, it is technically difficult to make its losstangent less than 0.01, and on the other hand, rolling resistanceincreases if its loss tangent exceeds 0.25. A more desirable range ofthe loss tangent is 0.07 to 0.25. Note that the loss tangent (tan δ)here is the one measured by using a viscoelastic spectrometer(manufactured by Toyo Seiki Seisaku-sho, Ltd.), with the conditions of afrequency of 20 Hz, an initial distortion of 10%, and a dynamicdistortion of plus or minus 2%.

Additionally, with regard to the rubber composition forming the rubberlayer 7 a at the outermost periphery, road noise having a frequencyaround 40 Hz is deteriorated if the JIS-A hardness is less than 70. Onthe other hand, if the JIS-A hardness exceeds 95, rolling resistance isdeteriorated while road noise having a frequency around 100 Hz isdeteriorated.

Moreover, with regard to the rubber composition forming the rubber layer7 a at the outermost periphery, if the breaking elongation is less than200% when measured at a temperature of 23° C. in a tensile test,durability is deteriorated. Although it is not required to particularlylimit an upper limit of the breaking elongation, the upper limit isabout 350% in reality. Note that the breaking elongation here is the onemeasured in compliance with JIS K6251.

As the rubber composition exhibiting the above physical properties, itis possible to use one obtained by blending 20 to 120 weight parts ofsilica and 0 to 60 weight parts of carbon black with 100 weight parts ofrubber. In a case where a blended amount of silica is out of the aboverange, it is difficult to obtain the above physical properties. In acase where carbon black is additionally blended, it becomes difficult toobtain the above physical properties if a blended amount of carbon blackexceeds 60 weight parts.

As silica, for example, dry-method white carbon, wet-method whitecarbon, colloidal silica, and precipitated silica can be cited. Thesekinds of silica may be used by one or in combination of two or more.

As base rubber, for example, natural rubber (NR), styrene-butadienecopolymer rubber (SBR) can be cited. These kinds of rubber may be usedby one or in combination of two or more. Additionally, to the rubbercomposition thus obtained, in addition to silica and carbon black, acompounding agent used regularly may be added. As the compounding agent,process oil, vulcanizer, vulcanization accelerator, antioxidant, andplasticizer can be cited.

In the pneumatic tire configured as above, the cross-sectional shapes ofthe bead filler 7 are defined, and additionally, while the two rubberlayers 7 a and 7 b formed of mutually different rubber compositions areprovided, the physical properties of two kinds of the rubbercompositions forming these rubber layers are defined. Consequently, itbecomes possible to reduce road noise without deteriorating durabilityand rolling resistance.

FIGS. 2 to 7 respectively show pneumatic tires according to otherembodiments of the present invention. In particular, each of FIGS. 2 to4 represents the one in which a height of a bead filler is maderelatively short, and each of FIGS. 5 to 7 represents the one in which aheight of a bead filler is relatively tall.

As shown in FIG. 2, the rubber layer 7 a at the outermost periphery maybe arranged so as to be closer to a main portion of the carcass layer 4than the rubber layer 7 b at the innermost periphery. As shown in FIG.3, the rubber layer 7 a at the outermost periphery may be arranged so asto be closer to a turned-up portion of the carcass layer 4 than therubber layer 7 b at the innermost periphery. As shown in FIG. 4, therubber layer 7 a at the outermost periphery may be arranged so as to bethrust into the rubber layer 7 b at the innermost periphery.

As shown in FIG. 5, the bead filler 7 may include a number of rubberlayers 7 a to 7 g respectively formed of mutually different rubbercompositions. Alternatively, as shown in FIG. 6, there may be aplurality of rubber layers 7 f to 7 h in the triangle portion of thebead filler 7. To the contrary, as shown in FIG. 7, there may be onlyone rubber layer 7 b in the triangle portion of the bead filler 7. Thus,in the present invention, it is possible to variously change adistribution of the rubber layers included in the bead filler, as longas the defined conditions are satisfied.

While the detailed descriptions have been given of the preferredembodiments of the present invention hereinabove, it should beunderstood that various modifications to, substitutions for, andreplacements with the preferred embodiments can be carried out as longas the modifications, the substitutions, and the replacements do notdepart from the spirit and the scope of the present invention defined bythe attached claims.

EXAMPLES

A conventional example, examples 1 to 9, and comparative examples 1 to 6were respectively manufactured as pneumatic tires with a tire size of205/65R15 in which only structures of their bead fillers are madevariously different. In the conventional example, each of the beadfillers includes only one rubber layer, and in the examples 1 to 9 andthe comparative examples 1 to 6, each of the bead fillers includes twokinds of rubber layers. Then, physical properties respectively of rubbercompositions forming the respective rubber layers of the bead fillersare variously different as shown in Tables 1.

For an upper-end position A of each of uniform-thickness portions of thebead fillers, entered is a periphery length measured along an innersurface of the tire from an upper end of each bead core to an upper endof a uniform-thickness portion of the bead filler. Provided that aheight H of a triangle portion of the bead filler is 35 mm, 1.6 timesthe height H is 56 mm, which is 60 mm if it is converted into aperiphery length. On the other hand, a periphery length measured alongthe inner surface of the tire from the upper end of the bead core to anedge of belt layers is 130 mm. Additionally, a lower-end thickness G ofthe triangle portion is 7.0 mm.

With regard to these test tires, road noise, rolling resistance anddurability were assessed by the following manners, and the resultsthereof are also shown in Tables 1.

Road Noise:

Each of test tires were mounted onto wheels of a rim size of 15×6½ JJrespectively, and then were installed to the four wheels of a passengercar in the class with a displacement of 3000 cc. Then, the vehicle wasrun on a paved road in a state where: an air pressure was 190 kPa; and aspeed was 60 km/h. Through a microphone installed at the center of rearsheets of the vehicle, sound pressure levels (in dB) at frequencies of40 Hz and of 100 Hz were measured. Results of the assessment are shownin differences from a standard value (of the conventional example). Anegative value means that road noise is lower than the standard value,and a positive value means that road noise is higher than the standardvalue.

Rolling Resistance:

Each of the test tires was mounted onto a wheel of a rim size of 15×6½JJ, and then rolling resistance thereof was measured, by using adrum-type tire testing machine, in a state where: an air pressure was190 kPa; a speed was 80 km/h; and a load was 4.6 kN. Results of theassessment are shown in index numbers where a rolling resistance valueof the conventional example is taken as 100. A higher value of the indexnumber means that rolling resistance is higher.

Durability:

Each of the test tires was mounted onto a wheel of a rim size of 15×6½JJ, and then durability thereof was measured, by using a drum-type tiretesting machine, in a state where: an air pressure was 190 kPa; a speedwas 80 km/h; and a load was as described below. The load was startedwith 88% of the maximum load and was increased by 13% each step, and atest was ended with 270% of the maximum load. Note that one stepcorresponded to two hours until the load reached 140% of the maximumload, and to four hours after the load reached 140% of the maximum load.Results of the assessment are indicated by “OK” for a case where thetest tire run to the end of the test and by “NG” for a case where thetest tire was destroyed at some midpoint of the test. TABLE 1Conventional Comparative Comparative Comparative Comparative exampleExample 1 example 1 example 2 example 3 Example 2 Example 3 example 4Height H of triangle 50 mm 35 mm 35 mm 35 mm 35 mm 35 mm 35 mm 35 mmportion Upper-end position A — 90 mm 90 mm 90 mm 50 mm 60 mm 125 mm  130mm  of uniform-thickness portion Thickness T of — 2.0 mm  2.0 mm  2.0mm  2.0 mm  2.0 mm  2.0 mm  2.0 mm  uniform-thickness portion tanδ(outermost — 0.15 0.26 0.15 0.15 0.15 0.15 0.15 periphery) tanδ(innermost 0.26 0.26 0.15 0.26 0.26 0.26 0.26 0.26 periphery) Hardness(outermost — 85 85 82 85 85 85 85 periphery) Hardness (innermost 95 9595 95 95 95 95 95 periphery) Elongation — 200% 200% 250% 200% 200% 200%200% (outermost periphery) Elongation 150% 150% 150% 150% 150% 150% 150%150% (innermost periphery) Road noise at 40 Hz 0.0 −1.0 −1.0 −1.0 0.20.0 −2.0 −2.2 Road noise at 100 Hz 0.0 −1.5 −1.5 −1.5 −3.2 −3.0 −0.1 0.2Rolling resistance 100 88 101 88 73 75 100 113 Durability OK OK OK NG OKOK OK OK Comparative Comparative example 5 Example 4 Example 5 example 6Example 6 Example 7 Example 8 Example 9 Height H of triangle 35 mm 35 mm35 mm 35 mm 35 mm 35 mm 35 mm 35 mm portion Upper-end position A 90 mm90 mm 90 mm 90 mm 90 mm 90 mm 90 mm 90 mm of uniform-thickness portionThickness T of 0.1 mm or 0.5 mm  3.5 mm  4.0 mm  2.0 mm  2.0 mm  2.0 mm 2.0 mm  uniform-thickness less portion tanδ (outermost 0.15 0.15 0.150.15 0.07 0.25 0.15 0.18 periphery) tanδ (innermost 0.26 0.26 0.26 0.260.26 0.26 0.26 0.26 periphery) Hardness (outermost 85 85 85 85 75 85 7090 periphery) Hardness (innermost 95 95 95 95 95 95 95 90 periphery)Elongation 200% 200% 200% 200% 200% 200% 200% 200% (outermost periphery)Elongation 150% 150% 150% 150% 150% 150% 150% 150% (innermost periphery)Road noise at 40 Hz Impossible 0.0 −2.0 −2.4 −0.9 −1.1 −0.3 −1.2 Roadnoise at 100 Hz to be −2.8 −0.2 0.3 −1.4 −1.6 −1.9 −1.1 Rollingresistance fabricated 78 98 101 76 99 83 92 Durability OK OK OK OK OK OKOK

As apparent from this Table 1, as compared to the conventional example,all of the examples 1 to 9 were able to reduce road noise withoutdeteriorating durability and rolling resistance. On the other hand, eachof the comparative examples 1 to 6 was found inferior in at least one ofdurability, rolling resistance and road noise as compared to theconventional example, because each of them did not satisfy a part of theconditions defined by the present invention.

1. A pneumatic tire, which includes a carcass layer mounted between apair of a left and a right bead portions; belt layers arranged on anouter periphery of the carcass layer located in a tread portion; and abead filler arranged along the carcass layer on an outer periphery of abead core in each of the bead portions, wherein: the bead fillerincludes a triangle portion abutting the bead core and having asubstantially triangular cross-sectional shape, and a uniform-thicknessportion extending from a top of the triangle portion in a radiallyoutward direction of the tire and having a substantially uniformthickness; a position of an upper end of the uniform-thickness portionis outward, in a radial direction of the tire, from a position 1.6 timesas high as a height of the triangle portion, and is at least 5 mm apartfrom the belt layers; a thickness of the uniform-thickness portion isthicker than 0.1 mm and thinner than 0.5 times a width of a lower end ofthe triangle portion; the bead filler includes a plurality of rubberlayers respectively formed of mutually different rubber compositions;and plural kinds of the rubber compositions forming these rubber layershave loss tangents (tan δ) gradually decreasing from the rubber layer atthe innermost periphery of the rubber layers toward the rubber layer atthe outermost periphery thereof, are harder than other rubbercompositions neighboring the bead filler and have breaking elongationswhose mutual differences are 50% points or less.
 2. The pneumatic tireaccording to claim 1, wherein: the rubber composition of at least onerubber layer among the plurality of rubber layers included in the beadfiller, except for the rubber layer at the innermost periphery, has aloss tangent (tan δ) of 0.01 to 0.25 when measured at a temperature of60° C., a JIS-A hardness of 70 to 95 when measured at a temperature of23° C., and a breaking elongation not less than 200% when measured at atemperature of 23° C. in a tensile test.
 3. The pneumatic tire accordingto any one of claims 1 and 2, wherein: the rubber composition of atleast one rubber layer among the plurality of rubber layers included inthe bead filler, except for the rubber layer at the innermost periphery,is a rubber composition obtained by blending 20 to 120 weight parts ofsilica and 0 to 60 weight parts of carbon black with 100 weight parts ofrubber.
 4. The pneumatic tire according to any one of claims 1 and 2,wherein: the bead filler is constituted of two rubber layersrespectively formed of mutually different rubber compositions.